Thanks! I think I have the external differences clear now. I & II both have the shutter lock/winder mode collar and lack the focus screen screw. They differ only in the largely cosmetic knurling on the rewind knob, which only type I has. Type III loses the shutter lock collar, but gains the focus screen screw. You suggest that the film advance mechanism change happened when type I was updated to II, though I might have guessed it would have happened when II was updated to III, at the same time as the other major external changes. But perhaps II was a transitional version, where removing the rewind knob knurling coincided with the first significant internal change to the mechanism?
It would be interesting to see under the bottom plate for versions I, II and III, if those views are available.